Seung-Mahn Lee

pH and Down Load Effects on Silicon Dioxide Dielectric CMP

Understanding the pH and down pressure effects is critical in elucidating the chemical and mechanical mechanisms in chemical mechanical polishing (CMP). This paper describes the variation in polishing rate by nonagglomerated silicon dioxide particles. The repulsive interaction force, solubility of amorphous silica, and total contact area at the pad-particles-wafer interface are important factors in determining polishing performance. In situ friction force measurements are used to detect the variation of interfacial contact during polishing. Surface finishes and interaction force of silica/silica were investigated using atomic force microscopy.

Effects of Particle Concentration in CMP

This paper reports on characterization of the surface coverage of particles by in-situ lateral friction force measurement during chemical mechanical polishing. The lateral friction force apparatus was made to operate close to real CMP conditions. For these experiments a sapphire wafer of constant surface roughness was used. For both 2psi and 4psi down force we observed increase in lateral friction forces with increasing solid loading. The lateral friction forces have been found to be significantly dependent on the contact area at the wafer-pad-slurry interface, thus showing that in-situ dynamic friction force changes in the surface coverage of particles. From these results, we conclude that the enhancement of frictional force is due to increased contact area at the wafer-pad-slurry interfaces. The lateral friction force measurement can provide an understanding of wafer-pad-slurry interactions.

Novel Method for the Formation of Large‐Grained, Silicon Thin Films on Amorphous Substrates

We describe a novel, solid-phase crystallization method for synthesizing large-grained, textured silicon films on amorphous substrates at relatively low processing temperatures ( 10 μm) for films crystallized by the SSC method. Hall measurements conducted on boron-doped films, annealed at 800°C, showed excellent hole mobility with values exceeding 180 cm 2 /V s, which was almost a factor of six higher than that found in polycrystalline films obtained from standard annealing procedures (annealing temperature ∼600°C for 28 h).

Selective Area Excimer‐Laser Crystallization of Amorphous Silicon Thin Films

Amorphous silicon films deposited on high quality fused-silica wafers were crystallized using an excimer laser. A unique configuration was used, which is referred to as selective-area crystallization. This was done by using surface masking of the film during standard laser irradiation. The resulting microstructure of the films was then characterized by transmission electron microscopy. The samples processed using the selective-area crystallization method resulted in one of three microstructures. Samples processed with the lower laser energy densities (200-300 mJ/cm 2 ) demonstrated a standard microstructure consistent with regular laser annealing (grain size ∼ 200 nm). Samples processed with higher energy densities (300-470 mJ/cm 2 ) demonstrated either enlarged grain growth (a factor of three greater than standard microstructure) or anomalous grain growth (unusual aspect ratio or geometry).

Increased Copper Outplating from Dilute HF Solutions on Microstructurally Modified Silicon Surfaces

Copper contamination during dilute HF (DHF) cleaning is one of the critical issues in silicon semiconductor device manufacturing. In this paper, copper outplating from DHF solution onto silicon surfaces was studied with transmission electron microscopy. It was found that copper ions preferentially deposited only at the amorphous silicon regions. A sharp circular boundary between the copper deposited and undeposited areas was obtained which was found to correspond to the amorphous-crystalline transition in the substrate. The deposited copper particles were face centered cubic polycrystalline structure with particle size ranging from 5 to 60 nm. Similar Cu deposition patterns were also found on other samples with scanning electron microscopy. The pattern formation of copper deposition was attributed to the preferential nucleation of copper in the amorphous regions.

Fundamental Studies on the Mechanisms of Oxide CMP

In this paper, results of studies on the addition of salt to a polishing slurry, in terms of its effect on slurry stability, SiO 2 polishing rate and surface roughness of the polished surface are presented. Three salts, viz. LiCl, NaCl and KCl were selected, and three concentrations were tested. Polishing rate measurements using these slurries show that adding salt leads to increased removal rate without affecting surface roughness significantly. Based on these results, we can say that the agglomerates formed by adding salt to the slurry are fairly soft and easily broken during the polishing process. In addition, turbidity and particle size measurements show that significant coagulation of the particles in the slurry occurs only at the highest salt concentration, and is fastest for LiCl and NaCl, with KCl showing the slowest coagulation. From these results, it can be concluded that the enhancement in polish rate is due to increased contact at the wafer-pad-slurry interface, and not due to formation of larger agglomerated particles in the slurry. This is because of reduced electrostatic repulsion between these three surfaces, due to the screening of their negative surface charge by the metal ions in solution, resulting in a higher wear rate.

Dynamic Contact Characteristics During Chemical Mechanical Polishing (CMP)

In chemical mechanical polishing (CMP), it is critical to understand dynamic contact at the pad-particles-wafer interface for desired CMP performance. The dynamic contact is dependent on process variables (platen velocity and down pressure) and particle characteristics (size and concentration), which in turn affect friction force. In this study, we have characterized the dynamic contact at the pad-particles-wafer interface as a function of platen velocity and down pressure. In situ lateral friction force measurements were carried out for silica slurry / sapphire wafer system in order to investigate the dynamic contact during polishing. As solids loading increases, the slope in the friction force vs. platen velocity curve changes from a negative to a positive value. Friction force increases with down pressure for different solids loading conditions. Consequently, friction force is determined as a function of down pressure and platen velocity, validating a dynamic contact mechanism during CMP.

Lateral Solid Phase Crystallization of Amorphous Silicon Under High Pressure

The authors have investigated a novel surface-seeded crystallization technique at low processing temperatures ({le} 550 C) and high pressures (10MPa {approximately} 25MPa) using polished polycrystalline diamond seeds. By controlling the high pressure, the nucleation and growth of silicon can be controlled to obtain improved quality silicon films on amorphous substrates at low temperatures. Depending on the annealing temperature and applied pressure, the orientation of crystallized silicon thin films varies as seen by x-ray diffraction and transmission electron microscopy results. In addition, crystallization of amorphous silicon thin films has effect on their roughness.